In most known phase shift measuring methods, the zero crossings of the a-c voltages to be compared are used for measuring the phase angles between the input and output voltages. The time between the zero crossings of the voltages is then measured. As an example we mention FIG. 5.8/12 from Steinbuch: "Taschenbuch der Nachrichtenverarbeitung" Springer 1962, p. 765.
If only the zero crossings of the a-c voltages to be compared are used for measuring the phase angles, superimposed interfering voltages may make it impossible to obtain an exact measurement of the phase angle. By attaching suitable filters, the a-c voltages to be tested can be freed as far as possible of interfering voltages, but the resulting phase shift is problematic. Also, the accuracy and constancy of the trigger circuit for determining the zero passages must satisfy the highest demands.
One object of the invention is to eliminate as far as possible the disadvantage of devices which constitute the known state of the art, particularly in the measurement of the phase angle between the input and output terminals of an inductive four terminal network. It has already been suggested (dissertation by Paul Vogel, University Basle 1972 "The use of active resonant circuits for measuring phase and damping") to connect the network to be measured into the feedback circuit of an R-C phase shifted oscillator, for example, (where the generated frequency depends on the phase shift of the network and of the R-C phase shifter), and to design the circuit so that certain oscillation conditions, as substantiated above, are satisfied.
In general the transmission function F(p) of the R-C phase shifter can be realized by various networks, for example, an all-pass filter can be used with the transmission function EQU F (p) = 1 - pT/1 + pT
wherein p is the differentiator p = d/dt and wherein T is the time constant of RC phase shifter.
But if a coil assembly (i.e. a transformer defining the network whose phase is to be measured) is provided in the feedback circuit of the oscillator, the voltage surges and phase rotations occurring in the self-resonant frequencies can likewise have the result that the above oscillation conditions are satisfied (Equations 1, 2). The oscillator can thus oscillate at different frequencies, if an all-pass filter is used as a phase shifter.